Integrated high-power tunable laser with adjustable outputs

ABSTRACT

A tunable laser that includes an array of parallel optical amplifiers is described. The laser may also include an intracavity N×M coupler that couples power between a cavity mirror and the array of parallel optical amplifiers. Phase adjusters in optical paths between the N×M coupler and the optical amplifiers can be used to adjust an amount of power output from M−1 ports of the N×M coupler. A tunable wavelength filter is incorporated in the laser cavity to select a lasing wavelength.

RELATED APPLICATIONS

The present application is a continuation claiming the benefit under 35U.S.C. § 120 of U.S. patent application Ser. No. 16/133,660 filed Sep.17, 2018 entitled “INTEGRATED HIGH POWER TUNABLE LASER WITH ADJUSTABLEOUTPUTS;” which is a continuation claiming the benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 15/383,555 filed Dec. 19, 2016and issued as U.S. Pat. No. 10,079,472 on Sep. 18, 2018 entitled“INTEGRATED HIGH POWER TUNABLE LASER WITH ADJUSTABLE OUTPUTS;” which isa continuation claiming the benefit under 35 U.S.C. § 120 of U.S. patentapplication Ser. No. 14/797,018, filed Jul. 10, 2015 and issued as U.S.Pat. No. 9,559,487 on Jan. 31, 2017 entitled “INTEGRATED HIGH-POWERTUNABLE LASER WITH ADJUSTABLE OUTPUTS;” which claims the benefit under35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No.62/023,483, filed Jul. 11, 2014 entitled “INTEGRATED HIGH-POWER TUNABLELASER WITH ADJUSTABLE OUTPUTS;” all of which are incorporated herein byreference in their entireties.

BACKGROUND Field

The present application relates to tunable lasers, optical amplifiers,and to optical communication systems.

Related Art

Tunable lasers conventionally consist of a tunable wavelength filter anda single optical gain medium inside a resonant laser cavity. A depictionof a conventional tunable laser 100 is shown in FIG. 1. The laser cavitymay include an intracavity beam 102 that reflects between ahigh-reflector end-mirror 105 and partially-transmitting mirror 140(referred to as an “output coupler”). The intracavity beam passesthrough the gain medium 110 and tunable wavelength filter 130 as itcirculates between the end-mirror and output coupler.

Such lasers normally have only one output beam 104, which emits from theoutput coupler 140. For example, the output coupler may transmit about10% of the optical power in the intracavity beam 102 outside the lasercavity to form the output beam 104. Conventionally, the amount of powercoupled outside the laser cavity cannot be adjusted while the laser isoperating. Instead, the laser must be shut off, and a different outputcoupler 130 installed and aligned.

Because a conventional laser contains one gain medium, the laser poweris limited by the saturation power of the gain medium 130. Once thesaturation power level is reached in the gain medium, no furthersubstantial increase in output power from the laser cavity can beachieved. To increase available laser power, a conventional techniquepasses the output beam 104 through an optical amplifier locateddownstream of the laser 100.

BRIEF SUMMARY

The present technology relates to tunable lasers, high-power lasers, andoptical amplifiers. A plurality of optical amplifiers may be integratedin parallel into a laser cavity. Additionally, the laser may include atunable filter and provide a plurality of power output ports, wherepower from each port is adjustable. According to some embodiments, alaser having a laser cavity may comprise a reflector at a first end ofthe laser cavity and an intracavity N×M coupler arranged to receivelight from the reflector at a first port and distribute the light to Noutput ports. The laser may further include Q optical amplifiersarranged to amplify light from at least some of the N output ports andat least one reflector arranged to reflect the amplified light back tothe N×M coupler. The number of optical amplifiers incorporated in thelaser cavity may be greater than or equal to two.

Methods for operating a tunable laser having integrated opticalamplifiers are also described. According to some embodiments, a methodof generating coherent light may comprise acts of reflecting light froma first reflector, and distributing the reflected light, with an N×Mcoupler, to N optical paths. The method may further include producingamplified light by amplifying light in at least two of the N opticalpaths, and returning the amplified light to the N×M coupler and firstreflector. A method of operating a tunable laser may also includeadjusting a phase of an optical signal in at least one of the N opticalpaths to adjust an output power from one of multiple power output portsof the tunable laser.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1 depicts a conventional tunable laser cavity;

FIG. 2 depicts a tunable laser that includes N optical amplifiersarranged in parallel, according to some embodiments;

FIG. 3A depicts an N×M coupler, according to some embodiments;

FIG. 3B depicts an N×M coupler, according to some embodiments;

FIG. 4 depicts a thermo-optic phase shifter, according to someembodiments;

FIG. 5 depicts a tunable wavelength filter, according to someembodiments;

FIG. 6A depicts a waveguide loop mirror, according to some embodiments;

FIG. 6B depicts a waveguide mirror, according to some embodiments;

FIG. 7A depicts butt-coupled waveguides with mode size adapting regions,according to some embodiments;

FIG. 7B depicts butt-coupled waveguides with mode size adapting regions,according to some embodiments;

FIG. 8 depicts a semiconductor optical amplifier, according to someembodiments;

FIG. 9 depicts a tunable laser that includes N optical amplifierscoupled to a coherent optical receiver and optical transmitter,according to some embodiments;

FIG. 10A and FIG. 10B depict an alternate embodiment for coupling Noptical amplifiers to N ports of an N×M coupler in a tunable lasercavity;

FIG. 11A depicts an embodiment of a coupled optical amplifier chip andsilicon photonics chip in which a laser cavity is distributed betweenthe two chips;

FIG. 11B depicts an embodiment of a coupled optical amplifier chip andsilicon photonics chip in which a laser cavity is distributed betweenthe two chips; and

FIG. 11C depicts an embodiment of a coupled optical amplifier chip andsilicon photonics chip in which a laser cavity is distributed betweenthe two chips.

DETAILED DESCRIPTION

The present technology pertains to tunable lasers that may be used inoptical communication systems, among other applications. Aspects of theapplication include apparatus and methods to provide a tunable laserthat includes a plurality of optical amplifiers in a parallelconfiguration and that can provide output power from multiple adjustablepower ports. Additionally, the tunable laser is readily scalable tohigher powers and additional power ports. According to another aspect ofthe application, methods of manufacturing a tunable laser of the typesdescribed herein are disclosed.

According to some embodiments, a tunable laser, such as tunable laser200 described below in connection with FIG. 2, may be microfabricatedand used in integrated optical systems, such as photonic integratedcircuits (PICs). The PICs may be used in optical communication systemsor optical coherent tomography systems, for example. In some cases, thetunable laser may be used for supplying an optical carrier wave and/orlocal oscillator to optical transmitters and receivers. In someembodiments, a tunable laser (such as tunable laser 200) may befabricated in a fiber-optic system, e.g., as a tunable fiber laser. Atunable fiber laser may include a plurality of fiber amplifiers arrangedin parallel and coupled into a laser cavity using fiber couplers.

The aspects and embodiments described above, as well as additionalaspects and embodiments, are described further below. These aspectsand/or embodiments may be used individually, all together, or in anycombination of two or more, as the application is not limited in thisrespect.

Referring to FIG. 2, a tunable laser 200 in accordance with an aspect ofthe present application may comprise an array of optical amplifiers230-1, 230-2 . . . 230-N (referred to collectively as 230 andindividually as 230-m) that are coupled to an intracavity N×M opticalcoupler 250 and to a first cavity reflector 207 at a first end of thetunable laser cavity. The N optical amplifiers may be coupled to the N×Mcoupler through a plurality of optical paths 212 and 222. In someembodiments, the optical paths may comprise integrated photonicwaveguides, e.g., fabricated from semiconductor and/or oxide material ona substrate. In some implementations, the optical paths 212 and/or 222may comprise fiber-optic waveguides. A second end of the laser cavitymay comprise a second reflector 205 arranged to reflect light backthrough the optical amplifiers 230. Additionally, the laser may includeP output power ports 260-1 . . . 260-P (referred to collectively as 260and individually as 260-m). As described below, in some embodiments Nand P are integers and P may be equal to or less than M−1.

Although FIG. 2 depicts N optical amplifiers coupled to N input ports ofthe N×M coupler, some embodiments may have fewer than N opticalamplifiers. For example, some embodiments may have Q optical amplifierscoupled to a portion of the N input ports of the N×M coupler, where Q isfewer than N. An input port that does not have an optical amplifiercoupled to it may be used as an intracavity power monitor in someembodiments.

The tunable laser 200 may further include a tunable wavelength filter130 and at least one intracavity phase shifter. At least one lasercavity optical path 224 extending between the N×M coupler 250 and thefirst cavity reflector 207 may be provided, and may include the tunablewavelength filter 130. In some embodiments, the laser cavity opticalpath 224 may include a phase shifter. In the illustrated embodiment, aplurality of intracavity phase shifters 240-1, 240-2 . . . 240-N areprovided (referred to collectively as 240, and individually as 240-m),one corresponding to each of the optical amplifiers 230. The phaseshifter(s) 240 may be located in the optical paths 222 connected to theN×M coupler 250. The wavelength filter 130 may be tuned to select alasing wavelength that circulates in between the first cavity reflector207 and second reflector 205, passing through the optical amplifiers230. The phase shifter(s) 240 may be tuned to adjust an amount of outputpower delivered from one or more of the power ports 260-1 . . . 260-P.The tuning (or adjusting) of the phase shifters may be dynamic (duringoperation of the laser) in some embodiments. Accordingly, a lasingwavelength for the tunable laser 200 may be selected by providing acontrol signal to the tunable wavelength filter 130. Additionally, powerfrom one or more of the power ports may be adjusted while the laser isoperating by providing one or more control signals to one or more of thephase shifters.

The tunable laser 200 may also include a wavelength locker 270. Thewavelength locker 270 may comprise an integrated photonic circuitconfigured to sense an operating wavelength of the tunable laser. Thewavelength locker 270 may comprise an interferometer, a Bragg gratingstructure, a resonator, or a combination thereof, and may produce asignal detected by wavelength locking circuitry 280 that is indicativeof a lasing wavelength for the tunable laser. An output from thewavelength locking circuitry 280 may be provided to the intracavitytunable wavelength filter 130, so as to stabilize an operatingwavelength of the tunable laser. In some embodiments, a wavelengthlocker 270 may be fabricated from a material having a low thermo-opticcoefficient. In some cases, the wavelength locker and/or at least aportion of the chip or chips on which the tunable laser is fabricatedmay be temperature controlled using a thermo-electric cooler or heater.

In operation, the tunable laser 200 may produce laser light thatreflects from the first cavity reflector 207, passes through the N×Mcoupler 250 where it is distributed to, and amplified by, the N opticalamplifiers 230, and then proceeds to the second reflector 205 where itis reflected back through the amplifiers and laser cavity. As the lightcirculates back and forth in the laser cavity, each of the N opticalamplifiers 230 contributes gain to the intracavity laser power.Additionally, a portion of the intracavity power is tapped out of thecavity through the P output power ports 260. In various embodiments, N,M, and P are integers. N may be greater than, or equal to, 2. M may beless than, equal to, or greater than N. P may be less than, or equal toM. In some embodiments, N=M, and P=N−1.

Providing an array of optical amplifiers 230 in parallel, rather thanjust one larger amplifier in a laser cavity, improves thermal andoptical performance of the tunable laser 200. The array spreads heatgenerated by the amplifiers 230 over a larger area, where it can bedissipated more easily. For example, an injection current to drive theamplifiers to obtain a given amount of power is spread over N separateregions of a substrate rather than being concentrated in a singleregion. An array of optical amplifiers also permits higher opticalsaturation power in the laser. In a semiconductor optical amplifier, theamount of available carriers per unit volume for optical gain may havean upper limit. Having multiple amplifiers in parallel increases theamplifier volume, while maintaining optical single-mode operation ineach amplifier, and therefore increases the amount of available carriersfor optical gain.

According to some embodiments, the optical amplifiers 230 may be locatedon a first semiconductor chip 210. The N×M coupler 250 and tunablefilter 130 may be located on a second semiconductor chip 220. The phaseshifters 240 may be located on the first or the second semiconductorchip. The first semiconductor chip 210 may comprise any suitable firstsemiconductor material, e.g., indium phosphide and/or any of its alloys(collectively referred to as indium phosphide), gallium arsenide and/orany of its alloys, or gallium nitride and/or any of its alloys. Thematerial of the first semiconductor chip 210 may be different from asecond semiconductor material of the second semiconductor chip 220. Forexample, the second semiconductor chip 220 may comprise silicon, silicondioxide, silicon oxynitride, and/or silicon nitride, and includeintegrated silicon photonic devices.

When a tunable laser, such as tunable laser 200, is distributed acrosstwo semiconductor chips, mode-size adapters may be formed at thejunction of the optical paths between the two semiconductor chips. Forexample, mode-size adapters may be formed at the ends of integratedwaveguides running to the edge of a chip. In the example of FIG. 2, modesize adapters 215-1, 215-2 . . . 215-N are provided (collectivelyreferred to herein as 215), one for each of the optical paths 212. Themode-size adapters may improve coupling efficiency of optical radiationfrom an optical path (e.g., a waveguide) 212 on the first semiconductorchip 210 to an optical path (e.g., a waveguide) 222 on the secondsemiconductor chip 222.

One example of an optical N×M coupler 350 which may be used as the N×Mcoupler 250 of FIG. 2 is depicted in FIG. 3A, although the variousaspects of the present application are not limited to only this type ofcoupler. In some implementations, an optical coupler may comprise amulti-mode interference (MMI) coupler or a star coupler. An opticalcoupler may comprise N first ports on one side of a coupling region 320and M second ports on a second side of the coupling region. In theillustrated example, N equals four, such that first ports 310-1 . . .310-4 (collectively referred to as 310) are provided, while M equals twosuch that second ports 330-1 and 330-2 (collectively referred to as 330and individually as 330-m) are provided. It should be appreciated thatother numbers of ports may be provided. In some embodiments, thecoupling region 320 may comprise an integrated slab waveguide in whichoptical modes entering from the N first ports 310 expand and interfereoptically before exiting through the M second ports 330. In someimplementations, an optical coupler 350 may be formed as an integratedsilicon optical device wherein the N and M ports and coupling region 320are fabricated as silicon waveguide structures. The N and M ports maycomprise single-mode optical waveguides each having transversedimensions between approximately 50 nm and approximately 700 nm. In someembodiments, a single-mode waveguide may have a height betweenapproximately 50 nm and approximately 300 nm and a width betweenapproximately 200 nm and approximately 700 nm. The coupling region 320may comprise a multimode slab waveguide, and have a same height aswaveguides of the N and M ports. A width of the coupling region may bebetween approximately 1 micron and approximately 50 microns.

In other embodiments, an optical coupler 350 may be formed using anysuitable semiconductor material, dielectric material, or materialcompositions. Material compositions may include metallic layers in someembodiments. Dielectric materials may include insulators such as oxidesor nitrides. Optical couplers formed from other materials or materialcompositions may have different dimensions than those listed above.

For an optical coupler 350, the first ports 310 may be referred to as“input” ports, and the second ports 330 may be referred to as “output”ports. However, an N×M optical coupler may exhibit reciprocity andoperate in both directions. For example, an optical coupler maydistribute light received at N “input” ports among M “output” ports. Thereceived light may have a distribution of N different intensities. Thelight distributed among the M output ports may have a distribution of Mdifferent intensities. The total of the output intensities may or maynot be approximately equal to a total of the input intensities,depending on the design of the optical coupler. In some implementations,the direction of the light may be reversed, so that the optical couplerdistributes light received at the M ports among the N ports having thesame distribution of N different intensities. Although FIG. 3A depicts a4×2 optical coupler, there may be any other number of first ports 310and second ports 330.

Another example of an N×M optical coupler 352 is depicted in FIG. 3B. Inthe illustrated example N equals two such that first ports 310-1 and310-2 are provided, and M equals four such that second ports 330-1 . . .330-4 are provided. These numbers are merely examples. According to someembodiments, an optical coupler may comprise a plurality of single-modeoptical waveguides that interact along their length. For example, two ormore waveguides may run parallel and in close proximity to each other atcoupling regions 315 and 325 (e.g., an optical directional coupler or anoptical adiabatic coupler). The coupling regions may be regions wheretwo or more waveguides are spaced near each other so that at least anevanescent field from one waveguide extends into at least one adjacentwaveguide. As an optical mode travels along a waveguide, power willcouple from one waveguide into at least one adjacent waveguide.

As noted above, one or more of the optical paths extending between theN×M optical coupler 250 and the N optical amplifiers 230 may include oneor more phase shifters 240. In some implementations, there is one phaseshifter in each optical path 222. A phase shifter 240-m may beconfigured to adjust a phase of an optical signal traveling along theoptical path 222, and to affect the optical interference of fields atthe N×M coupler 250. By adjusting the phase of an optical signal in oneor more optical paths 222, an amount of power emitted from the outputpower ports 260 and in the laser cavity optical path 224 can be altered.For example, adjusting a phase in one of the optical paths 222 canchange the way in which the optical fields interfere at the N×M coupler250 and deliver power to each of the M ports. As an example, accordingto one phase setting all of the intracavity power may flow through thelaser cavity optical path 224. Another phase setting may distribute someof the intracavity power among the power ports 260.

For the phase shifters 240 to affect optical interference in the N×Mcoupler 250 consistently over a wide wavelength range, the optical pathlengths between the N×M coupler and second reflector 205 through eachoptical amplifier may be approximately equal. In practice, the opticalpath lengths may differ, provided they do not differ by more than thetemporal coherence length of the laser radiation. Having differentoptical path lengths may result in wavelength dependence, and may beused in some embodiments to provide optical wavelength filtering in thelaser cavity. In some implementations, the phase shifters 240 may beadjusted by control signals that are varied manually and/orautomatically. For example, each power port 260-m may include an opticaltap and a power detector, so that an operator can provide a controlsignal to adjust the phase shifters to obtain a desired power ratio fromthe power ports. Additionally or alternatively, feedback circuitry orany suitable control circuit may provide control signals to the phaseshifters 240 responsive to detected power at one or more ports, so as tostabilize power from one or more ports 260. The feedback circuit may beany suitable circuit or may be implemented via digital signalprocessing. A feedback circuit may receive at least one power signalfrom a detector arranged to monitor a power from a power port, andprovide a control signal to a phase shifter 240-m to alter a phaseresponsive to the received power signal. The feedback circuit maycompare the received power signal to a second signal to determine avalue for the control signal.

FIG. 4 depicts a non-limiting example of a phase shifter 440 that may beused in a tunable laser, for example as a phase shifter 240-m of thetunable laser 200. According to some embodiments, the phase shifter 440may be a thermo-optic phase shifter, as described by M. R. Watts et al.,in “Adiabatic Thermo-Optic Mach-Zehnder Switch,” Opt. Lett. Vol. 38, No.5, 733-735 (2013), which is incorporated herein by reference. Suchthermo-optic phase shifters can achieve efficient optical phasemodulation of up to 2π in a length of waveguide less than 20 microns. Inother embodiments, the phase shifter 440 may comprise asemiconductor-based phase shifter that alters phase by current injectioninto a waveguide. For an optical fiber implementation, the phase shiftermay comprise piezoelectric material that stretches a length of fiber.Regardless of the type of phase shifter 440, it may be controlled by anelectrical bias to adjust the phase of an optical signal traversing theoptical path 222.

A thermo-optic phase shifter may include resistive elements 410 locatedadjacent an optical path (assumed to be a waveguide in this example)222. In some embodiments, there may be just one resistive element 410adjacent to the waveguide. A resistive element 410 may be located besideand/or above and/or below the optical waveguide. A resistive element maybe formed of a resistive semiconductor material, metal, or any othersuitable material that converts electrical current into heat. Athermo-optic phase shifter 440 may further include electricallyconductive traces that extend to a first terminal 425 and a secondterminal 427. The first and second terminals may be contact pads.Electrical current may be applied across the resistive element 410 viathe first and second terminals. As current flows through the resistiveelement, the resistive element may dissipate heat that couples to atleast a portion of the optical waveguide 222 and thereby change therefractive index within the optical waveguide. This change in refractiveindex can change the phase of an optical signal traveling through thewaveguide 222. A thermo-optic phase shifter may extend along a waveguideover a distance between approximately 2 microns and approximately 400microns, according to some embodiments. Other embodiments may includeother lengths.

The various aspects described herein are not limited to thermo-opticphase shifters 440. In some implementations, a phase shifter 240-m maycomprise an electro-optic phase shifter. An electro-optic phase shiftermay comprise a semiconductor junction (e.g., a p-n or p-i-n) formed in aportion of an integrated waveguide. The semiconductor junction may beconfigured to inject carriers into a region of the waveguide throughwhich an optical mode travels. The injection of carriers increasesoptical absorption and can change the refractive index in the waveguidethrough Kramers-Kronig relations applied to the optical absorption. Anelectro-optic phase shifter may extend along a waveguide over a distancebetween approximately 50 microns and approximately 800 microns,according to some embodiments. Other embodiments may include otherlengths.

FIG. 5 depicts a tunable wavelength filter 530 that may be included in ahigh-power, tunable laser cavity, according to some embodiments. Forexample, the tunable wavelength filter 530 may serve as the tunablewavelength filter 130 of FIG. 2. The tunable wavelength filter 530 mayinclude a pair of integrated photonic ring resonators 510, 520 locatedadjacent to a laser cavity optical path 224. The ring resonators 510,520 may be circular, elliptical, or have a race track pattern, and maybe of different sizes. Each ring resonator may have a free spectralrange that can be adjusted thermo-optically through resistive heatingelements 540. By adjusting the free spectral range of each ringresonator, it is possible to select a wavelength of an optical signalthat can couple to the first ring resonator 510 from the laser cavityoptical waveguide 224-1, couple to an intermediate waveguide 224-2,couple to the second ring resonator 520, and couple to an end waveguide224-3, where the optical signal travels to and reflects from the lasercavity reflector 207.

The ring resonators 510, 520 may be formed as integrated opticalwaveguides, according to some embodiments. The ring resonator waveguidesmay have a transverse profile approximately the same as a transverseprofile of the cavity optical waveguide 224-1, and described above. Insome embodiments, the ring resonator waveguides may be formed from asame material as the cavity optical waveguide 224-1 (e.g., silicon).

According to some embodiments, the first laser cavity reflector 207 maycomprise any suitable reflector that can be integrated on a PIC. Oneexample of a reflector is illustrated in FIG. 6A. According to thisembodiment, a reflector may comprise a waveguide loop mirror. Forexample, an end of the cavity optical path (e.g., waveguide) 224 mayextend into a loop 610 at an end of the laser cavity that circles backon the cavity optical waveguide. In some embodiments, the loop 610 maycomprise a single mode waveguide, having a same transverse profile andformed of the same materials as the cavity optical waveguide 224. Theloop may extend in any suitable shape, e.g., a teardrop shape.

Another example of a reflector 207 that can be implemented at an end ofa waveguide on a chip is depicted in FIG. 6B. According to thisembodiment, a reflector may comprise a multi-mode interference reflectorhaving an expanded region 630 of a waveguide 224. The expanded region630 may comprise a slab waveguide region or optical cavity into which anoptical mode traveling along the waveguide 224 may expand, opticallyinterfere, and reflect back into the waveguide 224.

According to some embodiments, a second reflector 205 at an opposite endof the laser cavity may be implemented as a reflective coating depositedon facets on optical waveguides 212. For example, the firstsemiconductor chip may be cleaved or cut, exposing facets of thewaveguides 212 that pass through the optical amplifiers. A reflectivecoating may then be deposited on the exposed facets. The reflectivecoating may comprise a multi-layer dielectric coating having highreflectivity at the lasing wavelength. In other embodiments, a pluralityof second reflectors may be used. For example, each waveguide 212 mayinclude a loop mirror or multi-mode interference reflector, so that thesecond reflector 205 comprises an array of reflectors.

As illustrated in FIG. 2, the optical amplifiers 230 may be located on afirst semiconductor chip 210 and the N×M coupler 250 may be located on asecond semiconductor chip 220. The optical amplifiers 230 may connect tothe N×M coupler through N optical paths 212, 222 that compriseintegrated optical waveguides. The integrated waveguides may, in someimplementations, be butt-coupled to one another at edges of thesemiconductor chips, as depicted in FIG. 7A and FIG. 7B. Suchbutt-coupled waveguides can provide efficient transfer of power from onewaveguide 212 on one chip to another waveguide 222 on an adjacent chip.Where the waveguides meet at the edge of the chip, there may bemode-size adapters 215. One example of a mode-size adapter 715 isdepicted in FIG. 7A.

In some implementations, a mode-size adapter 715 may comprise a portionof an integrated optical waveguide that changes in structure as itapproaches the edge of a semiconductor chip or a region where opticalcoupling to another waveguide will take place. For example, a waveguide212 for an optical amplifier 230-1 may expand gradually in size and/orcurve at a mode-size adapting region 720 near an edge of the firstsemiconductor chip 210. The expansion in size may increase a transversedimension of a waveguide up to 2 microns or more, in some cases. Theexpansion may allow the optical mode in the waveguide to expand in adirection transverse to the waveguide near the edge of chip, and therebymake coupling of the optical mode from one waveguide on the first chipto a second waveguide 222 on the second semiconductor chip 220 lesssensitive to misalignment between the waveguides.

Mode-size adapting regions 720, 730 of each waveguide may follow anysuitable curved path, such that an optical mode travelling along anoptical axis 703 in the first adapting region 720 and exiting the firstsemiconductor chip 210 is aligned with an optical axis of the secondadapting region 730 and makes an angle α with respect to a normal of afacet of each waveguide at the chip edges. The angle α may be betweenapproximately 5° and approximately 40°, according to some embodiments.Butt-coupling using such angled optical axes can reduce deleteriouseffects of potential reflections from the chip edges. For example,potential reflections from the chip edge may reflect into a directionthat is not readily coupled back into the optical waveguide 212. Thereflected light may be in a direction that is outside the numericalaperture of the waveguide, and therefore is not captured and guided bywaveguide 212. An optical adhesive or index-matching adhesive may beused in some cases to bond the butt-coupled waveguides. In someimplementations, the optical axes of the butt-coupled waveguides may benormal to the chip edges, and optical adhesive or index-matchingadhesive may be used to bond the butt-coupled waveguides. According tosome embodiments, an anti-reflection coating (e.g., a multi-layerdielectric stack) may be formed on facets of but-coupled waveguides toreduce interface reflections (e.g., when going from an InP chip in whichthe cladding may comprise InP to a Si chip in which the cladding maycomprise an oxide).

Another example of a mode-size adapter 717 is depicted in FIG. 7B. Insome embodiments, the optical waveguides may taper and reduce in lateraland/or vertical dimensions in the mode-size adapting regions 722, 732 atedges of the respective chips. In some implementations, a transversedimension of a waveguide may reduce to about 50 nm. By reducing atransverse dimension of an optical waveguide, an optical mode within thewaveguide is confined less strongly and expands out into the surroundingdielectric or air as the waveguide becomes smaller. This can increasethe lateral dimension of an optical mode traveling along the waveguideas it approaches the edge of the semiconductor chip.

According to some embodiments and referring again to FIG. 2, the opticalamplifiers 230 may comprise any suitable type of optical amplifier. In afiber-optic system, an optical amplifier may comprise an erbium-dopedfiber, for example. When implemented in a PIC, an optical amplifier maycomprise a semiconductor optical amplifier (SOA). In some embodiments,an SOA 800 may have a structure as depicted in the elevation view ofFIG. 8. For example, an SOA may be formed on a semiconductor substrate805, which may be a silicon (Si) substrate or an indium phosphide (InP)substrate, though other semiconductor substrates may be used in otherembodiments. There may, or may not, be a dielectric or insulating layer810 (e.g., an oxide or nitride layer) on the substrate. For example, thesubstrate may comprise a semiconductor on insulator (SOI) substrate,according to some embodiments. The insulation layer 810 may be betweenapproximately 50 nm thick and approximately 4 microns thick.

In some implementations, a semiconductor optical amplifier 800 maycomprise InP material and include a first n-doped InP base layer 820formed on the substrate. The base layer 820 may be between 100 nm thickand approximately 2 microns thick. A buffer layer 825 comprising n-dopedInP may be formed on the base layer 820. The buffer layer 825 may beformed by epitaxial growth or ion implantation, according to someembodiments, and may be between approximately 5 nm and approximately 50nm thick. An intrinsic layer 830 of InP may be subsequently grownepitaxially on the buffer layer. The intrinsic layer 830 may be betweenapproximately 50 nm and approximately 200 nm thick, according to someembodiments. A p-doped layer 840 may be formed on the intrinsic layer toform a p-i-n junction. The intrinsic layer 830 and p-doped layer 840 maybe epitaxially grown. The SOA 800 may additionally includeelectron-blocking and hole-blocking layers (not shown) in someembodiments. A first electrical contact (not shown) may be formed on thep-doped layer 840, and a second electrical contact may connect to thebase layer 820 so that a current can be applied across the p-i-njunction.

According to some implementations, a SOA, such as SOA 800, may bepatterned in a waveguide structure. A cross-section of the waveguidestructure may have a profile as depicted in FIG. 8. An optical mode maybe confined to the waveguide and pass primarily through the intrinsicregion 830 of waveguide where carrier recombination and opticalamplification can occur. Although FIG. 8 depicts a ridge waveguide, someembodiments may include a buried waveguide (e.g., a waveguide comprisingsemiconductor material surrounded on two or more sides by a dielectrichaving a lower refractive index).

Although a semiconductor optical amplifier is described as beingindium-phosphide based (which includes alloys of InP) in connection withFIG. 8, other materials may be used in other embodiments for a SOA. Forexample, a SOA may comprise gallium arsenide and/or its alloys. In someimplementations, a SOA may comprise gallium nitride and/or its alloys.In some embodiments, a SOA may comprise alloys of indium-aluminum.

According to some embodiments, a tunable laser that includes a pluralityof optical amplifiers may be implemented in a PIC for opticalcommunications, as depicted in FIG. 9. A PIC 900 may comprise thetunable laser 901 that includes, for example, a 4×4 optical coupler 250,four SOAs 230-1, 230-2, 230-3, and 230-4, four mode size adapters 215-1,215-2, 215-3, and 215-4, and four phase shifters 240-1, 240-2, 240-3,and 240-4. The tunable laser 901 may include three power output ports904-1-904-3 in addition to a laser cavity path 244 that directsintracavity power to a tunable wavelength filter 130 and a reflector207. The laser cavity path 244 may be the same in nature as thepreviously described path 224. Laser light from a first power port 904-1may couple to a coherent receiver 902. This laser light may provide alocal oscillator signal for the coherent receiver. Laser light generatedby the tunable laser 901 may also be provided through two other powerports 904-2, 904-3 to an optical transmitter 903.

The coherent receiver 902 may be formed on a same semiconductor chip 220as a portion of the tunable laser 901, or may be formed on a differentsemiconductor chip. In some embodiments, a coherent optical receiver mayinclude an optical surface coupler 910 that is configured to receivesignal light, on which information is encoded, from an optical fiber andcouple the signal light into integrated optical waveguides of thecoherent receiver 902. The coherent receiver may further include one ormore integrated coherent receiver photonic circuits 920-1, 920-2 thatprocess the received signal light and produce a plurality of electricalsignals that can be detected through contact pads 915. A localoscillator signal provided from the tunable laser 901 may be split withan optical splitter 912 to provide a local oscillator signal for each ofthe integrated coherent receiver circuits 920-1, 920-2. The integratedcoherent receiver circuits 920-1, 920-2 may include phase-diverse andpolarization-diverse photonic circuits.

The optical transmitter 903 may comprise a pair of nested Mach-Zehnderinterferometers 950. A nested Mach-Zehnder interferometer may include aplurality of optical splitters 912 and a plurality of thermo-optic phaseshifters 940. A nested Mach-Zehnder interferometer may also includehigh-speed electro-optic phase modulators 952. The nested Mach-Zehnderinterferometers may be used for quadrature signal modulation and/or dualpolarization modulation of optical signals. An optical transmitter 903may further include an output surface coupler 980 that is configured tocouple optical radiation from one or more waveguides to an opticalfiber.

According to some embodiments, a nested Mach-Zehnder interferometer 950may also include a 2×2 optical coupler 914 from which one exit port mayprovide an optical reference signal to an on-chip photodetector. Thephotodetector may convert the optical reference signal to an electricalsignal that can be detected at a signal pad 915. The electrical signalcan be monitored to determine relative powers from the two nestedMach-Zehnder interferometers 950.

Although a tunable laser has been described as having a portionincluding optical amplifiers formed on a first semiconductor chip 210and a second portion formed on a second semiconductor chip 220, in someimplementations a tunable laser may be formed on a single semiconductorchip, as depicted in FIG. 10A for tunable laser 1000 on chip 1010. Anelevation view of the structure shown in FIG. 10B. For example, an N×Moptical coupler 320 and integrated optical waveguides 310-1 . . . 310-4,330-1 . . . 330-4 may be formed in a first semiconductor layer 1022 on asubstrate 1005. In some implementations, the first semiconductor layermay comprise a silicon semiconductor layer (e.g., a silicon-on-insulatorlayer). Additionally, the phase shifters 240-1 . . . 240-4 for one ormore of the N input ports to the coupler 320 may be formed on the firstsemiconductor layer 1022. A second layer 1024 of semiconductor material,for example InP may be formed over the first semiconductor layer 1022 asdepicted in FIG. 10B. According to some embodiments, the second layer ofsemiconductor material may be formed by a bonding process. For example,a wafer bonding and etch-back process may be employed as described inU.S. Pat. No. 9,020,001, which is incorporated herein by reference.

Semiconductor optical amplifiers 230 and their respective waveguides 212may be formed in the second semiconductor layer 1024. In someembodiments, an insulating dielectric, for example an oxide, 1030 may bedeposited between the layers. An insulating layer may also be depositedas an overlayer to passivate the device. Power from the semiconductoroptical amplifiers 230 on the second layer may couple to the lower ports(e.g., silicon waveguides) 310 by evanescent coupling. In this manner,the power in the laser cavity can travel from one cavity reflector 205through the semiconductor optical amplifiers, into the underlyingsilicon waveguides, and through the N×M optical coupler 320 to anothercavity mirror 207 (not shown) coupled to one port 330-m of the opticalcoupler 320.

In some embodiments, it may be preferable to form the optical amplifiers230 on a different substrate from other components of the tunable laser.FIGS. 11A-FIG. 11C depict embodiments in which the SOAs may be formed ona first substrate or semiconductor chip 210 and coupled to a secondsemiconductor chip 220 on which the N×M coupler, phase shifters, andtunable wavelength filter are located. In some implementations, the SOAsmay be formed on a “process side” or “device side” 1110 of the firstsemiconductor chip 210. As shown in the configuration 1101 of FIG. 11A,the first chip may be flipped and bonded to a first sub-mount 1115. Suchflip-chip bonding can improve heat dissipation from the SOAs. Forexample the sub-mount may comprise a material (e.g., aluminum nitride)having higher thermal conductivity than the material of the firstsemiconductor chip 210 (e.g., indium phosphide). The first sub-mount1115 may be bonded to a base mount 1105. The second semiconductor chip220 may then be aligned and bonded to the base mount. In someembodiments, the components on the second semiconductor chip 220 areformed on a process side 1120. In some embodiments, the secondsemiconductor chip 220 may be manipulated with a positioning device andits alignment to the first chip adjusted until a correct alignment isachieved. According to some embodiments, correct alignment may bedetected by monitoring optical power transferred from one chip to theother. In some cases, a magnified image of the chip interface may beviewed or processed to determine correct alignment. The magnified imagemay be obtained through a combination of optical and electronicmagnification using optical lenses and CCD or CMOS imaging array. Oncealigned, an epoxy or UV-curable adhesive 1130 may then be cured to affixthe second semiconductor chip 220 and preserve the alignment. In someembodiments, a UV curable adhesive or optical adhesive may additionallybe located between the first semiconductor chip 210 and the secondsemiconductor chip 220 to provide both adhesion and index matchingbetween the optical paths (e.g., waveguides) 212, 222 on each chip.

According to some embodiments, both the first semiconductor chip 210 andthe second semiconductor chip 220 may be flip-chip bonded to a basemount 1105, as depicted in configuration 1102 of FIG. 11 B. In somecases, one or both of the chips may be solder bonded (e.g., using bumpbonds) to the base mount 1105. For example, the solder may be heatedbefore bonding, the chips aligned, and then the solder may be cooled tobond the chips and preserve the alignment. In some embodiments, aUV-curable or optical adhesive may be used additionally between thechips and/or between the chips and base mount 1105 to aid in permanentlyfixing the chips after alignment has been achieved.

According to some implementations, the first semiconductor chip 210containing the SOAs may be flip-chip bonded to the second semiconductorchip 220, as depicted in configuration 1103 of FIG. 11C. According tothis embodiment, the second semiconductor chip 220 may include a trench1150 or other receiving feature to receive the first semiconductor chip210. For example, the trench may have a depth between approximately 500nm and approximately 10 microns, such that optical paths (e.g.,waveguides) 212, 222 on the two chips become essentially coplanar whenthe chips are bonded together. The chips may be aligned and bonded usingsolder bonding and/or adhesive bonding as described above.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, and/or methods described herein, if suchfeatures, systems, articles, materials, and/or methods are not mutuallyinconsistent, is included within the scope of the present disclosure.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified.

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. The transitional phrases “consisting of” and “consisting essentiallyof” shall be closed or semi-closed transitional phrases, respectively.

I claim:
 1. An optical system comprising: a semiconductor opticalamplifier (SOA); wherein the SOA is formed on a first semiconductorchip; wherein the first semiconductor chip is flip-chip bonded to afirst sub-mount; wherein the first sub-mount is mounted to a base mount;wherein the first semiconductor chip comprises a first set of opticalpaths; wherein the SOA is coupled to at least a path of the set ofoptical paths; and a second semiconductor chip mounted right side up onthe base mount; wherein the second semiconductor chip is aphoto-integrated circuit (PIC) facing upward; where the firstsemiconductor chip is coupled to the second semiconductor chip; whereinthe second semiconductor chip comprises a second set of optical paths;wherein at least one optical path of the first set of optical paths isconnected to at least one optical path of the second set of opticalpaths.
 2. The optical system of claim 1 wherein the second semiconductorchip has a tunable wavelength filter.
 3. The optical system of claim 1wherein the SOA is formed on a process side of the first semiconductorchip.
 4. The optical system of claim 1 wherein the first sub-mountcomprises aluminum nitride and has a higher thermal conductivity of amaterial of the first semiconductor chip and enables improved heatdissipation.
 5. The optical system of claim 4 wherein the material ofthe first semiconductor chip is indium phosphide.
 6. The optical systemof claim 1 wherein the second semiconductor chip is aligned and bondedto the base mount.
 7. The optical system of claim 6 wherein an adhesiveaffixes the second semiconductor chip to preserve alignment of the firstsemiconductor chip and the second semiconductor chip.
 8. The opticalsystem of claim 1 wherein the second semiconductor chip comprises atrench to receive the first semiconductor chip.